Extensible workspaces.yaml format

[Daniel Playle <dan playle gmail com>]

Hey all,

The workspaces state file, `workspaces.yml`, does not currently afford
much in the way of extensibility, providing only a mapping between
sources and open workspaces.

Further enhancements around workspaces will require the ability to
store more state. As Chandan has previously proposed, we can change
the format of `workspaces.yaml` to be more general, and therefore be
more extensible.

To assist with parsing the proposed format, a version attribute is
introduced, allowing a new parser of the `workspaces.yaml` file to
accept both formats, thereby not breaking any existing workspaces when
a user upgrades to BuildStream using the proposed format.

Current Format:
```
alpha.bst:
  0: /workspaces/bravo
  1: /workspaces/charlie
```

Proposed Format:
```
version: 1
build-elements:
  alpha.bst:
    sources:
      0:
        path: /workspaces/bravo
      1:
        path: /workspaces/charlie
```

I'd like to know if this proposal is something that the community
thinks would be valuable?

Daniel